In delta modulating and other similar systems, the digitally-encoded signal stream (1's and 0's) represents the slope of each segment of the encoded analog signal. A digital 1 means the analog slope is positive, and the decoder reconstructs the analog signal by increasing it a predetermined amount. A digital 0 causes the decoder to decrease the analog signal by the same amount. The digital signal is arrived at by comparing the analog input signal with the reconstructed analog signal produced by decoding the digital signal. A 1 is generated if the input is less than the reconstructed signal, and a 0 if the input is greater. Typically, the difference between the input and reconstructed signals is first computed, and then the difference is applied to a comparator amplifier that generates a digital 1 if the difference is one polarity and a digital 0 if the other polarity.
The reconstructed analog signal is made up of a sequence of positively or negatively-sloped segments, and thus has a sawtooth appearance as compared to the smoother input signal. It is desirable, particularly in acoustic devices, to keep the sawtooth, or quantization, noise inaudible. Conventionally this has been approached by raising the frequency of the encoding clock high enough to place the frequency of the sawtooth outside the audible frequency range and by adjusting the segment slopes of the reconstructed signal so that they are about the same magnitude as the slope of the input.